Ferrules are utilized to form leak-free fluidic connections between two conduits. Ferrules are often employed in applications entailing small-scale fluid flows, such as analytical instruments and microfluidic devices, and thus may be sized to join small-bore conduits such as capillary tubing or fluidic fittings. As one example, a ferrule may be utilized to join the end of a capillary-scale chromatographic column to a fitting that is part of or communicates with an analytical detector or the sample inlet of an analytical measurement device containing a detector such as, for example, a mass spectrometer. Ferrules are typically monolithic articles composed of a metal, graphite, or a composite such as graphite-polyimide. The capillary tubes engaged by ferrules are often composed of fused silica with a polyimide coating. The body of a typical ferrule is axisymmetric and defines an internal bore through which the tube to be sealed is inserted. At least a portion of the ferrule, or “nose” portion, is often conical.
In a typical application, forming a fluidic seal entails inserting the capillary conduit through the ferrule and then inserting the ferrule and tube into the bore of a fluidic fitting. Alternatively, the ferrule may be inserted into the fitting bore first and the conduit then inserted into the ferrule. The ferrule is inserted far enough that a sealing region on the outside surface of the conical nose portion of the ferrule comes into contact with a conical inside wall of the fitting. The ferrule is then translated axially against the conical inside wall of the fitting typically by engaging the axial end of the ferrule opposite to the nose portion with a nut. The nut has an internal or external thread that is brought into mating engagement with a corresponding thread formed on the fitting, such that rotation of the nut causes axial translation of the ferrule. Consequently, the ferrule at its sealing region is compressed against the conical inside wall of the fitting, and the inside wall defining the internal bore of the ferrule is compressed and swaged against the tube.
In making the fluidic seal, it is easy to apply too much crushing force against the tube, causing the tube to break or otherwise collapse. Particularly when the tube is composed of a material susceptible to cracking such as fused silica, the tip of the ferrule tends to bite into the capillary conduit with excessive force, cracking or breaking it and causing premature failure of a fluidic seal that otherwise seemed to be well-made and leak-free.
In certain gas chromatography applications, it is desirable to ramp the temperature of fittings and other components at fast rates. This requires low thermal mass fittings and ferrules. As ferrules become smaller, it not only becomes difficult to sense when a ferrule is over-compressed, it also increases the difficulty of adding features to prevent over-swaging due to limited machining capabilities. Similarly, for a low thermal mass fitting, the compression nut and threaded portion of the fitting can be eliminated, being replaced by a press fit feature on the ferrule and fitting. While this lowers the thermal mass of the fitting, it also increases the likelihood of over-swaging the ferrule due to the force of the press-fit masking the compression force from the ferrule. Finally, repeated thermal cycling can make the ferrule become loose over time, requiring features to prevent this from occurring.
Therefore, there is a need for providing a ferrule configured to eliminate or at least significantly reduce the tendency to cause sharp crushing of the ferrule tip against the capillary conduit during formation of the fluidic seal.